Cobaloximes, reactions

FIGURE 18.6. Cobaloxime reaction, (a) Details of the reaction in Figure 18.5. (b) Difference electron-density maps show that, above 293K, the racemization occurs, giving two possible positions for the methyl group. 

Theoretical investigations have been used to model coenzyme Bi2. While important conclusions may be drawn, the work has not been extended with CCT in mind. Due to computational limitations, the initial work was limited to the triaminomethylcobalt111— amide system.67 Force fields specifically designed to do molecular mechanical calculations on cobalamins68 and cobaloximes69 are relevant for conformational studies but cannot elucidate the mechanisms of cobaloxime reactions because they do not account for electronic effects. Computational investigation of... 

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

Branchaud and coworkers have used cobaloximes as alkyl radical precursors for the cross-coupling reaction with nitronates.57 This method is very useful for producing branched-chain monosaccharides, as shown in Eq. 5.39.57b... 

Microwave-assisted Diels-Alder cycloaddition reactions using water-soluble aquo-cobaloxime complexes have been reported by Welker and coworkers [198]. Many more examples of microwave-assisted cycloaddition processes leading to heterocycles are described in Section 6.24. 

Steric factors play an important role in reactions of bromomethylaqua-cobaloxime with substituted imidazoles - 1-methylimidazole reacts at approximately the same rate as imidazole itself, but the presence of an alkyl substituent in the 2-position of the incoming imidazole reduces the rate dramatically (72). 

Detailed kinetic studies confirmed a two-stage reaction for the cobaloxime(II)-catalyzed autoxidation of this system in methanol (54,55). First, within about 30 s, the reaction reached steady-state conditions via reversible oxygenation of Co(II) to the corresponding... 

The major cobaloxime species, Com(HDTBC), was present at an approximately constant concentration level in the slow stage, in which the following additional reaction steps were considered ... 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

Certain cobalt(II) complexes such as the cobaloxime XXVIIa and the corresponding boron fluoride XXVIIb (L is a ligand) terminate propagating chains by a transfer reaction called... 

The extent of the above so-called side reaction was determined by considering the decrease in the absorbance of the chloride band. We found that with the stoichiometric amount of chloro-(pyridine)cobaloxime(III), the side reaction was much more prominent than with the catalytic amount. Nevertheless, no appreciable unsaturation (the absence of CF=CF2 around 1790 cm1)39 was indicated in the IR spectra in both cases, implying that the undesired two-electron transfer process was not involved. 

It should be noted that the similar addition did not succeed when a stoichiometric amount of chloro(pyridine)cobaloxime(III) was utilized. We found that the reaction of PCTFE did occur, but with a significant level of the side reaction (broad bands in 3100 -3700 and 1500 - 1800 cm-i regions in the IR spectrum. The rationale of the result was that a large number of PCTFE radicals were rapidly produced at one time. Some of these radicals were, therefore, available to react with THF concurrently with the desired addition to allyltributyltin. 

Once again, it should be noted that the reductive condition in which the stoichiometric amount of chloro(pyridine)cobaloxime(III) was used, not only aided the addition of PCTFE to styrene, but also promoted the side reaction. In the IR spectrum of the resulting polymer, the reduction in the absorbance of the chloride band at 972 cm-i seemed to be too large, and the incorporation of oxygen in 3300 - 3600 cm-i region was also evident. The situation similar to the case of allyltributyltin might be applied in that THF possibly also took part in the reaction. 

Since the stoichiometric condition did provide the addition of PCTFE to ethyl acrylate but along with the side reaction, we expected that the chemospecificity of the addition might be improved by changing the condition to the catalytic one. This assumption was based on the result observed in the blanks — the less the amount of chloro(pyridine)cobaloxime(III) used, the lower the degree of the side reaction. 

It was shown that by employing the catalytic quantity of chloro(pyridine)cobaloxime(III) (Equation 6), the chemo-specificity of the addition was enhanced. In accordance with the IR spectrum of the functionalized PCTFE (Figure 6), the carbonyl band at 1736 cm- was much larger while the reduction in the absorbance of the chloride band at 972 cm- was much smaller, compared with those observed in the stoichiometric condition. However, the side reaction was not completely suppressed as the bands due to the side reaction were still present. 

Similar to the case of ethyl acrylate, the chemospecificity of the addition of PCTFE to methyl methacrylate was improved by using a catalytic amount of chloro(pyridine)cobaloxime(III). The IR spectrum of the resulting polymer showed the carbonyl band at 1731 cm-i. Although the bands due to the side reaction were also present, the decrease in the absorbance of the chloride band at 974 cm-i became less obvious, inferring that the extent of the side reaction was lowered. 

A number of model compounds have been used to study the reactions of B,2. The ligands of these compounds are usually oximes and Schiff s bases and pyridine is a common axial base. Cobalt-bisdimethylgiyoxime, Co(dmg)2 (68) is the most popular model and is commonly called cobaloxime (81). 

The dehydration and deamination reactions appear to operate in a parallel fashion and will be considered together. Schrauzer and Silbert propose a base-catalyzed cleavage of the carbon-cobalt bond in the B12-catalyzed diol dehydration reaction as shown in Fig. 17 (81), based on demonstrated lability of the beta hydrogens in alkyl cobaloximes with electronegative groups in this position. 

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